1. Field of the Invention
This invention relates to an apparatus for conveying a substrate such as a vitreous photomask or a reticle, and in particular to an automatic conveying apparatus suitable for incorporation into an exposure apparatus, a cleaning apparatus or the like.
2. Description of the Prior Art
In recent years, the tendency of semiconductor elements toward minuteness and high density has advanced and various performances required of apparatuses for manufacturing them, particularly, exposure apparatuses, have become severe year by year. In a reduction projection type exposure apparatus (a so-called stepper) having a high resolving power and a high superposing system, a pattern is reduction-projected onto a semiconductor wafer by the step-and-repeat system by the use of a reticle which provides the negative of pattern transfer and therefore, if a foreign particle adheres to the reticle, there is a problem that all chips on the wafer become defective. For this reason, apparatuses for conveying the reticle fully automatically without touching the reticle when the reticle is mounted onto or removed from the apparatus have been put into practical use and have achieved a great result in the field of production of semiconductor elements. Such a conveying apparatus, with an example of the reticle case mountable on the conveying apparatus, is disclosed in U.S. Pat. No. 4,422,547. In such a conventional conveying apparatus, there has been a disadvantage that various operations of delivering the reticle are complicated with the reproducibility of positioning taken into account and a long conveyance time is required. Also, in order to prevent foreign particles from adhering to the reticle, a protective frame (pellicle frame) for stretching a high molecular weight transparent pellicle spaced apart by several millimeters from the surface of the reticle has come to be adhesively secured to the reticle, and the reticle with such a pellicle could not be conveyed by the conventional apparatus. Where the transfer of a pattern is to be effected by the use of a reticle with a pellicle, it is substantially ensured that there is no foreign particle directly adhering to the surface of the reticle, but a relatively large foreign particle adhering to the surface of the pellicle greatly affects the transfer of the pattern. So, it is efficient in the manufacture of semiconductor elements to use, for a reticle without a pellicle, a strict foreign particle inspecting device capable of discriminating between the sizes of foreign particles by a resolving power of the order of 2-3 .mu.m, and to use, for a reticle with a pellicle, a pellicle inspecting device for inspecting only a large foreign particle on the pellicle, for example, only resolvable dust of 100 .mu.m or more. Where both of such foreign particle inspecting device and pellicle inspecting device are retrofitted to a stepper, the conveying apparatus thereof must equally convey both of a reticle with a pellicle and a reticle without a pellicle, and this has led to a problem that high-speed conveyance cannot be accomplished as long as the conventional conveying method is used.